Professor Jae-Hwang Lee and his graduate students in our Mechanical and Industrial Engineering (MIE) Department have collaborated with other researchers at Washington University in St. Louis to further develop pioneering work on promising new nanocomposites that can be tailored as revolutionary ballistic armor significantly stronger and lighter than current armor materials. Lee and his collaborators authored a January 9 paper on their work in the high-profile materials science journal Nano Letters.

The study reported in Nano Letters continued the work on nanomaterial armor described previously in a 2014 paper published in Science magazine by Lee and his associates which, among other results, reported that the nanometer-thin graphene under study surpassed steel tenfold in bulletproofing effectiveness.

As Lee and his collaborators explained in their recent Nano Letters paper, they have for the first time demonstrated the extreme dynamic properties of ultrathin, freely-suspended, nanocomposite films by using a supersonic micro-bullet to test this incredibly strong, tough, and lightweight new material suitable for body armor and other similar uses. The paper is titled: “Extreme Mechanical Behavior of Nacre-Mimetic Graphene-Oxide and Silk Nanocomposites.”

Lee and associates said that “This study evidently demonstrates that the morphologies of nanoscale constituents and their interactions are critical to realize scalable high-performance nanocomposites using typical nanomaterial constituents having finite dimensions.”

Lee’s Nano Engineering Laboratory at UMass Amherst focuses on the rational design of various functional materials, including metamaterials, with emphasis on energy, defense, and bio applications through 2D/3D nano-structuring of polymers, metals, ceramics, and more. “Our quest is driven by the acknowledgement that future material innovation will rely on the development of novel materials based on tailored thermal, mechanical, and photonic responses of materials,” explained Lee.

Biological materials have the ability to withstand extreme mechanical forces due to their unique multilevel hierarchical structure. “Here, we fabricated a nacre-mimetic nanocomposite comprised of silk fibroin and graphene oxide that exhibits hybridized dynamic responses arising from alternating high-contrast mechanical properties of the components at the nanoscale,” as the Nano Letters paper explained.

The background of this research, according to the Nano Letters paper, is that nanocomposites, comprised of materials with distinct mechanical properties and tailored interfaces between the components, have received wide attention over the last two decades due to their synergistically improved properties compared to the individual components. Nanocomposites are promising for the development of lightweight ballistic armor materials, where dissipation of a projectile’s massive kinetic energy with a limited areal density (or mass per unit area) is required. The behavior of nanocomposites contrasts radically to that in conventional structural composites, in which irreversible structural damage via dynamic failure processes are generally accompanied by cracking, fragmentation, shear, and delamination.

In this respect, typical traditional composites that have micro- or larger scale material phases still tend to exhibit characteristic failure mechanisms of individual constituent materials because a material’s intrinsic failure mechanism is governed at the submicrometer scale. By contrast, in addition to their superior load transfer effciency, which is attributed to the large interfacial area between phases, composites with nanoscale phases or nanocomposites exhibit a hybridized failure mechanism for enhanced antiballistic performance.

“As a ﬁller for nanocomposites,” as the Nano Letters paper deduced, “graphene oxide (GO) stands out among reinforcing nanoﬁller materials due to its attractive characteristics, including high elastic modulus, low density, high water solubility, and good mechanical ﬂexibility. As a matrix, we employed silk ﬁbroin (SF), a material that forms one of the toughest natural ﬁbers, as well as a good candidate for both a binder and matrix.”

The combination of GO and SF forms a nacre-like “brick-and- mortar” arrangement, known for its remarkable strength and toughness.

The paper went on to say that the dynamic mechanical behavior of these nanocomposites was assessed through a microscale ballistic characterization using a 7.6 μm diameter silica sphere moving at a speed of approximately 400 m/s. The volume fraction of GO in these composites was systematically varied from 0 to 32 volume percent to quantify the dynamic effects correlating with the structural morphologies of the GO flakes.

“Specific penetration energy of the films rapidly increases as the distribution of graphene oxide flakes evolves from non-interacting, isolated sheets to a partially overlapping continuous sheet,” the paper observed. “The specific penetration energy of the nanocomposite at the highest graphene oxide content tested here is found to be significantly higher than that of Kevlar fabrics and close to that of pure multilayer graphene.”

Lee collaborated on the article with Amir Kazemi-Moridani of the UMass MIE department, Wanting Xie of the MIE and Physics Departments at UMass, and four researchers from the Department of Mechanical Engineering and Materials Science at Washington University in St. Louis. (February 2018)